Controlled Synthesis and Infrared Emission Properties of Core-Shell TiO₂ Hollow Microspheres

With the growing demand for advanced passive cooling technologies in fields such as building energy efficiency, thermal protection of electronic devices, and personal thermal comfort, radiative cooling materials have garnered considerable attention due to their ability to achieve cooling without external energy input. In this study, TiO₂ hollow microspheres with a core-shell structure were successfully synthesized via a solvothermal method using TiCl₄ as the titanium source and (NH₄)₂SO₄ and CO(NH₂)₂ as structure-directing agents. The effects of reaction temperature (120-200 °C) and reaction time (0.5-36 h) on the morphology, crystal phase, specific surface area, pore structure, and infrared optical properties of the microspheres were systematically investigated. The results indicate that all prepared samples consisted of anatase-phase TiO₂, with the microstructure significantly influenced by the synthesis conditions. An increase in reaction temperature promoted the transition from solid to hollow structures; the microspheres exhibited the most regular morphology and the largest specific surface area at 180 °C. Prolonging the reaction time facilitated the Ostwald ripening process, leading to a more complete hollow structure at 24 h. Infrared optical performance analysis revealed that all samples exhibited high emissivity approaching 100% in the 8-15 μm wavelength range, attributed to the intrinsic lattice vibration absorption of TiO₂. In the 3-8 μm range, however, the emissivity was strongly modulated by the microstructure. Samples synthesized at 180 °C for 12-24 h demonstrated stable emissivity characteristics owing to their dense shells, uniform particle size, and well-defined hollow structures. This study elucidates the intrinsic relationship between microstructural evolution and infrared emission performance in TiO₂ hollow microspheres, providing a theoretical foundation and process optimization strategy for their application in radiative cooling coatings, device thermal protection, and personal thermal management textiles.